In modern clinical analyzers fluids such as bodily fluids can be tested by various clinical chemical and immunochemical methods. Many analytical methods require precise pipetting operations so as to attain satisfactory analytical accuracy. Usually, pump-controlled probes are used for aspirating and discharging fluids.
For performing pipetting operations, the probe tip must reliably be placed in the fluid. In terms of minimizing the danger of cross-contamination and facilitating probe cleaning, it often is desirable to position the probe tip just below the fluid surface. Generally, when adding or removing fluid, the probe tip can either be kept stationary with respect to the sample vessel or can be lifted or lowered so as to keep it in a dedicated position relative to the fluid surface within the fluid.
However, in many cases, fluid surfaces are not exactly known or can greatly vary from one sample vessel to another. Therefore it is necessary to detect the fluid surface in order to accurately position the probe before starting a pipetting operation.
Generally, the detection of fluid surfaces can be based on various physical principles. One method is to detect light beams directed towards and reflected from the fluid surface. By detecting the time of travel, the distance between the probe tip and the fluid surface can be calculated.
Another method frequently used in sample processing is based on detecting a characteristic change of an electrical property of the probe when the probe is brought in or out of physical contact with the fluid. Specifically, one known technique detects the change of the electric resistance of the probe when the probe tip dips into the fluid. However, in order to obtain reliable results, the fluid should be in galvanic contact with electric ground which often is not the case and therefore this technique cannot be satisfactorily used in many cases. Fluid surface detection using resistance changes of the probe is described for example in U.S. Pat. No. 5,843,378A.
Another technique is based on the application of high-frequency voltage signals to the probe which, e.g., are in a range of from 1 MHz to 1 GHz so as to generate electric impedances sensitive for surface detection. However, fluid surface detection based on high-frequent impedance measurements requires sophisticated technical equipment and is rather cost-intensive. Due to the specific operating conditions, this technique is not appropriate for use in clinical analyzers. Furthermore, electric interference effects can result in a low electromagnetic compatibility of the analyzer. Fluid surface detection based on high-frequency impedance measurements, is described for example in WO 2000019211 A1, U.S. Pat. No. 5,049,826 A, U.S. Pat. No. 5,365,783 and U.S. Pat. No. 4,818,492A.
Yet another technique is based on a change of the electric capacitance of the probe when the probe is brought in or out of physical contact with the fluid. For this purpose, the probe is repeatedly charged by periodic electric signals, with the capacitance of the probe being measured by analyzing the discharging current. Typically, low-frequent voltage signals, lower than 1 kHz, are used to avoid electric impedances. In the patent literature, fluid surface detection based on a capacitance change, is described for example in EP 89115464 A2 and U.S. Pat. No. 7,150,190 B2.
Using the capacitance technique, a change of the capacitance of the probe can be observed when the probe hits the fluid surface. However, depending on various parameters such as sample volume (in clinical analyzers, samples typically have volumes of a few Milliliters (mL) orless), design and material of the sample vessel containing the sample and the surrounding conditions thereof, the change of the electric capacitance is very small. Typically, the change of capacitance of the probe is some ten Femtofarads (10−15 F) or less. Moreover, the measurement is likely to be disturbed by external influences such as static electric capacities with respect to adjacent probes and/or other electrically conductive parts neighbouring the probe. Further disturbances can be caused by dynamic electric capacities usually arising between moving electrically conductive components. Accordingly, when moving the probe, in particular, in a quick or irregular manner relative to electrically conductive parts, such as metallic components, dynamic electric capacities can be generated. As a matter of fact, such static and/or dynamic parasitic effects can be in the order of Picofarads (10−12 F) which is much larger than the capacitance change of the probe caused by bringing the probe in or out of physical contact with the fluid. Hence, fluid surface detection in clinical analyzers based on a capacitance change of a probe may not yield reliable results.
Therefore, it is desirable to improve conventional systems and methods for detecting the fluid surfaces of samples which are based on a change of the electric capacitance of probes when being brought in or out of physical contact with fluids.